In recent years, from the purpose of global environmental protection, resins which are easily decomposed under the natural environment are watched and studied in the world. As the resins which are easily decomposed under the natural environment, biodegradable polymers represented by aliphatic polyesters, such as polylactic acid, polyglycolic acid, poly(3-hydroxybutyrate), polycaprolactone, etc., are known.
Above all, polylactic acid is a polymer material that is high in biological safety and environmentally friendly because it is made of, as a raw material, from lactic acid obtained from a plant-derived raw material, or a derivative thereof. For that reason, utilization as a general-purpose polymer is investigated, and utilization as films, fibers, injection molded articles, and the like is investigated.
Recently, paying attention to easy decomposability of those resins and water solubility of decomposed monomers, practical use for excavation technology in the oil field is investigated (Patent Literatures 1 to 3). In this application, it is required that after keeping the weight and shape of a resin in hot water for a fixed period of time, the resin is quickly decomposed (see FIG. 1). However, in general, since aliphatic polyesters and the like are inferior in hydrolysis resistance, though they are usable up to a medium temperature of about 120° C., there is involved such a problem that they are immediately decomposed in high-temperature hot water (see FIG. 2), so that a desired performance cannot be exhibited.
Slowly decomposable resins, such as aromatic polyesters, etc., are not quickly decomposed even in hot water (see FIG. 3), and furthermore, there is involved such a problem that monomers generated by decomposition react with other components of the foregoing application and are deposited in water (Patent Literature 4). With respect to the high temperature, there are various definitions, such as “127° C. to 193° C.” described in the report: U.S. Shale Gas, published in 2008 by Halliburton Company, “149° C. or higher” described in Oil and Gas Review, 2002.5, published by Japan Oil, Gas and Metals National Corporation, etc., and in general, the high temperature is considered to be higher than “125° C. to 150° C.”. Incidentally, in the present invention, a temperature higher than 135° C. is referred to as “high temperature”.
Meanwhile, in order to enhance the hydrolysis resistance of aliphatic polyesters and the like, there is already proposed a method in which a hydrolysis regulator, such as a carbodiimide compound, etc., is used, and an acidic group generated at the early stage and by decomposition in the resin is sealed, thereby inhibiting the hydrolysis (Patent Literatures 4 to 6).
The acidic group generated by hydrolysis of the aliphatic polyester, such as a carboxyl group, etc., becomes an autocatalyst to promote the hydrolysis, and therefore, it is confirmed that by immediately sealing this by a carbodiimide compound or the like, the hydrolysis resistance under the moist heat environment at about 50 to 120° C. is enhanced.
However, with respect to the hydrolysis inhibition in hot water at a higher temperature than 135° C., there are not made sufficient investigations from the viewpoints of resin or hydrolysis regulator.
In the light of the above, it is the actual situation that a resin composition exhibiting a desired performance as shown in FIG. 1 in hot water at a higher temperature than 135° C. has not been obtained yet in the excavation technology in the oil field.
In addition, in the application of the excavation technology in the oil field, it is required that after keeping the weight and shape of a resin in hot water under a chemically severe condition, such as an acidic or basic condition, etc., for a fixed period of time, the resin is quickly decomposed (see FIG. 5). However, in general, since aliphatic polyesters and the like are inferior in hydrolysis resistance, though they are usable in approximately neutral hot water, they are quickly decomposed in strongly acidic or basic hot water (see FIG. 6), so that there is involved such a problem that a desired performance cannot be exhibited.
In addition, slowly decomposable resins, such as aromatic polyesters, etc., are not quickly decomposed even in strongly acidic or basic hot water (see FIG. 3), and furthermore, there is involved such a problem that monomers generated by decomposition react with other components of the foregoing application and are deposited in water.